LEDs are rapidly replacing incandescent bulbs, fluorescent bulbs, and other types of light sources due to their efficiency, small size, high reliability, and selectable color emission. A typical forward voltage drop for a high power LED is about 3-4 volts. The brightness of an LED is controlled by the current through the LED, which ranges from only a few milliamps to an amp or more, depending on the type of LED. For this reason, LED drivers typically include some means to control the current.
In some applications, such as backlights for large televisions, many LEDs of various colors, such as red, green, and blue, are used to achieve the desired brightness and overall color temperature. It is common to connect LEDs in series, since the current through all the LEDs in series will be the same. If the LEDs in a string are matched, each LED in the string will have similar light emission characteristics. For a color LCD display, there may be one driver for a string of red LEDs, a second driver for a string of green LEDs, and a third driver for a string of blue LEDs, where each string has a specified current to achieve the desired white point. Serial strings of LEDs may also be connected in parallel so as to limit the required driving voltage level and provide redundancy.
In some large display applications, there may be 25 or more LEDs connected in series, requiring a driving voltage of about 90 volts (about 25×3.5 volts). Such high voltages require components with a breakdown voltage well in excess of 90 volts.
FIG. 1 illustrates a conventional simple driver 10 for a string of LEDs, LED1-LEDn. Assume n=25. There may be many parallel strings of LEDs, all connected to the same high voltage, provided by a high voltage (HV) regulator 12, where each string may have a separate current controller. The conventional HV regulator 12 converts an input voltage Vin to a regulated voltage of a predetermined level. The input voltage Vin may be any voltage, such as a low battery voltage or a rectified wall outlet voltage. In the example of FIG. 1, it is assumed the regulator 12 outputs a DC voltage of 90 volts or more. The regulator 12 may be a buck or boost regulator, depending on the required input and output voltages.
To control the current through the LEDs, for brightness control and to avoid damage, the LEDs are coupled to ground via an FET 14 and a low value resistor 16. The current through the LEDs produces a voltage drop across the resistor 16, which is used as a feedback voltage Vfb to detect the current. The gate voltage of the FET 14 is adjusted by a current regulator 18 so that the feedback voltage Vfb matches a fixed reference when the FET 14 is turned on. Thus, the value of the resistor 16 can be used to set the maximum current through the LEDs.
To control the brightness of the LEDs, the average current through the LEDs is controlled by pulse width modulation (PWM), where the duty cycle (on time vs. total time) of the pulses determines the average current. For example, if the duty cycle were 50%, the average current would be half of the instantaneous current when the FET 14 is on. Thus, the perceived brightness of the LEDs would be, for example, half the brightness of the LEDs when fully on. The PWM signal is typically generated by a processor or other controller and changes when a different brightness is desired. The PWM frequency may be, for example, 1 kHz, so the light flicker is not perceptible.
When the FET 14 is on, the voltage across the FET 14 is very small, since the FET 14 acts like a short circuit. However, when the FET 14 is off, the voltage across the FET 14 is approximately the full drive voltage of 90 volts, since there is very little voltage drop across the LEDs due to no current flowing through the LEDs. This requires a very large FET that can withstand a breakdown voltage well in excess of 90 volts. Such large FETs are relatively expensive and less efficient than smaller FETs.
It would be desirable to drive a string of LEDs using a high voltage but where the FET (or other type of switch) that controls the current does not need a high breakdown voltage to support the full drive voltage.